Opinion: What we know about the cyanobacterial bloom affecting beaches on Waiheke Island

By Dr Laura Biessy, Microaglae Team Leader

10 May 2024

Waiheke Island’s beautiful beaches are beloved by locals and visitors alike, but for the past few summers people have been disappointed by the presence of a stinky and hazardous black sludge that has appeared on shore.

This sludge had not been a dominant feature of the local marine ecosystem of Waiheke, but beginning in November 2022 following severe and wild weather events, hundreds of tons of unidentified mats washed up and decomposed on the beach, in particular on Shelley and Blackpool beaches. Over the 22/23 summer, Auckland Council removed close to 400 tonnes of this material and nearby residents began experiencing headaches as well as eye and throat irritations leading the Council to begin investigations.

As Cawthron Institute is an important centre in Aotearoa New Zealand for research into toxic algae, Auckland Council asked us to identify the species and test for toxins. After genetic testing, we identified the dominant component of the cyanobacterial mats as Okeania spp., a species of benthic marine cyanobacteria normally found in tropical regions. They can grow loosely attached to seagrass, sand and/or bottoms and form mats that wash ashore.

Beginning in November 2023, large amounts of the same cyanobacterial mats started to wash up on the beaches of Waiheke Island and this continued for several months over the summer. They were also recorded at Kawakawa Bay on the other side of the Tāmaki Strait in February, four months after the start of the bloom.

My role as our Microalgae Research Team Leader at the Cawthron Institute is to identify and understand different species of microalgae, the toxins they produce and how they interact with the ecosystem around them. Presently I’m doing some further taxonomic analyses to identify the exact species responsible for these blooms on Waiheke Island, or understand if this is a new species that has not been described before. 

Credit: Cawthron Institute – Okeania mats washed ashore on Waiheke Island and decomposed to form a black sludge.

Credit: Cawthron Institute – Okeania biofouling off Waiheke Island.

Cawthron-FW-Water Quality Monitoring-Laura Biessy

Image: Dr Laura Biessy, Microalgae Team Leader

Both years, the Okeania mats tested positive for lyngbyatoxin-A, a toxin with acute dermatoxic effects collectively referred to as seaweed dermatitis or swimmer’s itch, as well as eye and respiratory irritations. Cawthron is currently testing shellfish collected from the affected beaches on Waiheke Island for lyngbyatoxin-A accumulation, as this could impact human and ecosystem health.
Unfortunately, there is a lack of research surrounding the ecology, toxicity and impact on the environment of benthic cyanobacterial species such as Okeania. We are currently exploring different options for public funding we could access to enable this work.

I am also currently trying to isolate and culture this species of algae in Cawthron Institute’s Culture Collection of Microalgae to enable future research, in particular into the ecology of the species, its preferred growing conditions (temperature, salinity, nutrients, etc.) and habitat. These answers would allow us to predict when future blooms might occur and what could be done to prevent or manage them.

One important thing to note is that our marine environment is changing all the time, and there are a number of stressors that can impact the delicate balance of these ecosystems, including activities on land that introduce pollutants, and increasing water temperatures as a result of global warming. There is a lot of uncertainty about how these stressors will compound and affect our oceans and coastal areas, but it is fair to say we will see the emergence of new species in our local ecosystems. The more we can do now to get ahead of potential problems by predicting what these species might be and understanding their ecology, the better prepared we will be to protect our unique and precious marine environment.

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